Chemistry / Unit 1 Chem 0091 short lesson, about 20 seconds
The Meter
Captions available
The idea
The meter, SI's unit of length, is the distance light travels in a vacuum during a fixed fraction of a second.
In practice
Because the defining constant is light's speed, any lab with an accurate clock can reproduce it without a bar.
Where people slip
Students often picture the meter as a physical bar in a vault — true until 1960, not today. (NIST/BIPM, Aug 20, 2026.)
Your place on the trail
Every lesson rests on earlier ones and opens later ones. Tap any to jump.
Builds on
Unlocks
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